An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor may be roughly classified into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor (CIS). During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion is also reduced, thereby resulting in a reduction in image quality. Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion is also reduced due to diffraction of light called airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on and/or over the readout circuitry has been made (referred to as a “three-dimensional (3D) image sensor). The photodiode is connected with the readout circuitry through a metal interconnection.
Since both the source and the drain in both sides of the transfer transistor are heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated. Also, because a photo charge does not readily move between the photodiode and the readout circuitry, a dark current is generated and/or saturation and sensitivity is reduced.